This invention relates to determination of nuclear magnetic resonance properties of formations surrounding an earth borehole and, more particulary, to a well logging method and apparatus for determining the nuclear magnetic resonance longitudinal magnetization decay of formations surrounding an earth borehole.
The longitudinal relaxation time constant of the formations, and/or the distribution thereof, can be obtained from the longitudinal magnetization decay.
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in conventional NMR operation the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, which is called the longitudinal relaxation time constant or spin lattice relaxation time constant. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation time constant, T2, called the transverse relaxation time constant or spin-spin relaxation time constant. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The net precessing magnetization decays with a time constant T2 because the individual spins rotate at different rates and lose their common phase. At the molecular level, dephasing is caused by random motions of the spins. The magnetic fields of neighboring spins and nearby paramagnetic centers appear as randomly fluctuating magnetic fields to the spins in random motion. In an inhomogeneous field, spins at different locations precess at different rates. Therefore, in addition to the molecular spin-spin relaxation of fluids, spatial inhomogeneities of the applied field also cause dephasing. Spatial inhomogeneities in the field can be due to microscopic inhomogeneities in the magnetic susceptibility of rock grains or due to the macroscopic features of the magnet.
A widely used technique for acquiring NMR data, both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then a one hundred eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to refocus. By repeatedly refocusing the spins using one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear, and the train of echoes is measured and processed. The transverse relaxation time constant, T2, or the distribution of T2""s, can be obtained using this technique. The determination of the longitudinal magnetization decay and of T1, however, remains difficult. A source of this difficulty is the very low signal-to-noise ratio inherent in detecting the feeble magnetic moment of nuclei.
The traditional pulse method of measuring the longitudinal relaxation time constant (T1) is the so-called inversion recovery method (see Carr et al., Phys. Rev. 94, 630 (1954)). In this method, a 180 degree pulse is applied to a nuclear spin system, followed by a recovery time and then a 90 degree read-out pulse. The amplitude at a convenient point on the resulting free induction decay is measured, and the spin system is then allowed to recover to equilibrium by waiting approximately five times T1 before applying the next two-pulse sequence. Many such cycles are required since the spin-lattice relaxation time is found by correlating the various recovery times with the associated free induction decay (FID) amplitudes. While this technique can provide an accurate measure of T1 (and, with further processing, the T1 distribution), the pulse sequence is very time consuming.
It is among the objects of the present invention to provide an apparatus and method that can determine the longitudinal magnetization decay and relaxation time constant of formations surrounding an earth borehole, with improved time and cost efficiency.
A form of the present invention provides a well logging technique and apparatus whereby the longitudinal magnetization decay and the longitudinal relaxation time constant (T1) of formations surrounding an earth borehole can be measured directly after a single RF pulse or other suitable perturbation in direction and/or magnitude of the magnetization of the spins. In embodiments of this form of the invention, the longitudinal magnetism (that is, magnetism parallel to the static magnetic field) is sensed, in general, by a magnetic flux detector, and specifically by a superconducting flux detector. SQUIDs are the most sensitive flux detectors, but there are other flux detectors (flux gate magnetometers, conventional coils and amplifiers, etc.) which can, in principle, be used.
Superconductors have been employed in the design of electromagnetic sensors that are extremely sensitive. The heart of a superconducting sensor is the Superconducting Quantum Interferometric Device (SQUID), which can be envisioned as a very sensitive converter of magnetic flux to voltage. Typically, a measurement circuit will be arranged so that the detected signal results in a current flow in a loop that is inductively coupled to the SQUID. SQUIDs do not generate high fields, and they do not carry high currents. They are mechanically robust and compact, and are therefore relatively easy to cool. SQUIDs have been proposed for use in borehole logging but have not become commercially prevalent for a number of reasons, one such reason being the difficulty of devising suitable and practical logging applications for SQUIDs. [See, for example, J. Jackson, New NMR Well Logging/Fracture Mapping Technique With Possible Application Of SQUID NMR Detection, Soc. Of Explor. Geo., Tulsa, Okla., pp. 161-165, 1981.]
In accordance with an embodiment of the method of the invention, there is disclosed a technique for determining the nuclear magnetic resonance longitudinal magnetization decay of formations surrounding an earth borehole, comprising the following steps: providing a logging device that is moveable through the borehole; applying, from the logging device, a static magnetic field in the formations to align spins in the formations in the direction of the static magnetic field; producing, from the logging device, a tipping pulse for tipping the direction of the spins with respect to the static magnetic field direction; and detecting, at the logging device, the time varying magnitude of the spin magnetization as said magnetization returns toward the static magnetic field direction; the longitudinal magnetization decay being determinable from the detected time varying magnitude of the spin magnetization. [As background, see Sager, Kleinberg, and Wheatley, Phys. Rev. Lett., 39, 1345 (1977), and Sager, Kleinberg, and Wheatley, J. Low Temp. Phys. 32, 263 (1978), regarding laboratory application of SQUID detected signals using a 180 degree pulse and monitoring of longitudinal magnetization for NMR investigation of 3He.]
In a preferred embodiment of the method of the invention, the step of detecting the time varying magnitude of the spin magnetization includes providing a magnetic flux detection system for producing a number of output signals representative of successively sampled values of the magnetic flux over a period of time. The magnetic flux detection system includes a magnetic flux detection sensor (or antenna), and a magnetic flux sensing circuit that preferably includes a SQUID. In a form of the preferred embodiment, at least part of the magnetic flux detection system is in a cooled enclosure.
In accordance with a feature of an embodiment of the invention, an all-metal pressure-tight housing is provided for the flux detection system. This facilitates thermal shielding (e.g. vacuum dewaring) for cooled components, and is possible because the quasistatic nature of the signal (a few hundred Hertz bandwidth) is below the frequency at which the metal housing becomes opaque to magnetic fields. For higher frequency operation, the portion of the metal housing in front of the flux detection antenna (e.g. receiver coil) can have a reduced wall thickness.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.